This invention relates to an imaging apparatus having an imaging drum to movably position sheet material such as thermal imaging material, dye donor sheets and direct writing plates during image formation. More specifically the present invention relates to mechanisms for loading, holding and releasing sheet material on an imaging drum.
In the art of image generation it is often necessary to form an image on a sheet material such as a thermal media or a plate. Typically the sheet material is secured to an imaging drum and the imaging drum is rotated while a print head forms an image on the material. This can be done by transferring dye or ink to a sheet of material or by modifying the sheet of material. The task of attaching sheet material to the vacuum imaging drum is often rigorous because it requires precise positioning of the sheet material on the imaging drum. Once that sheet material is properly positioned on the drum it is necessary to hold the sheet material in place during imaging operations. Various mechanisms are known in the art to help secure sheet material such as thermal media, dye donor materials and direct write plates to an imaging drum.
For example, it is known use mechanical clamping mechanisms located at the outer surface of the drum. Such mechanical clamping mechanisms must be robust enough to resist the forces generated by the high speed rotation of the drum. These mechanisms can be hand actuated or they can be automatically actuated as is shown in U.S. Pat. No. 5,678,486. Such mechanisms are often complex, and have a clamping force that does not adaptively increase with the speed of drum rotation.
It is also known to temporarily fix the media and direct write plates onto the outer surface of the drum using magnets. These magnets are attached on top of the media or direct write plates and the force of the magnetic attraction between the magnet and the outer surface holds the media or direct write plates in contact with the drum. It will be appreciated that the magnetic attraction between the magnet and the outer surface must provide sufficient magnetic attraction to resist the centrifugal force of the drum as it rotates. Because the centrifugal force increases with drum rotation speed, it is necessary to use more powerful magnets to secure the media to the drum in order to withstand the centrifugal force created by increased rotational speed. It will be recognized that a point is reached where it becomes impractical to use magnets that have sufficient attractive force to withstand the aforementioned centrifugal force.
In commonly assigned U.S. Pat. No. 5,268,708 a dual chamber vacuum imaging drum is used to help controllably position the dye donor media and the thermal media on the surface of an imaging drum. One chamber applies vacuum that holds the lead edge of the dye donor material. Another chamber controls vacuum which holds the trail edge of the thermal print media to the vacuum in imaging drum. With this arrangement, loading a sheet of thermal print media and dye donor material requires that the image processing apparatus feed the lead edge of the thermal print media and dye donor material into position just past the vacuum ports controlled by the respective valve chamber. Then vacuum is applied, gripping the lead edge of the dye donor materials against the vacuum imaging drum surface. Unloading the dye donor material or the thermal print media requires the removal of vacuum from the same chamber so that in edge of the thermal print media or the dye donor material are freed in project out from the surface of the vacuum imaging drum. The image processing apparatus deposits an articulating skive into the path of the free edge of the donor material to lift the edge further and feed the dye donor material to a waist bin or output tray.
Thus, while the presently known and utilized mechanisms for attaching sheets of material to an imaging drum of an image processing apparatus are commercially viable, a need exists for an imaging drum having an improved mechanism for securing sheet material to a vacuum imaging drum. Further, a need exists for an imaging drum that can secure sheet material even at high rates of drum rotation.
According to a feature of the present invention, an imaging apparatus is provided for forming images on sheet material. The imaging assembly comprises a print head for forming images on the sheets and an imaging drum having an outer surface. A motor is also provided for rotating the imaging drum. A material clamp is mounted to the imaging drum having a retainer positioning a retaining surface radially outward of the outer surface and forming a space therebetween. A slide is movable within the space between an outer radial position where outward radial movement of the slide is blocked by the retaining surface and an inner radial position distant from the retaining surface. A magnet is provided having a magnetic field biases the slide toward the outer radial position. Rotation of the drum creates centrifugal force that further biases the slide toward the outer radial position.
According to another embodiment of the present invention, an imaging drum assembly is provided for use in an imaging apparatus for forming images on sheet material. The imaging drum assembly comprises a rotatable imaging drum having an outer surface adapted to attract material to the drum. A material clamp mounted to the drum having a retainer positioning a retaining surface radially outward of the outer surface and forming a space therebetween, A slide is movable within the space between an outer radial position where outward radial movement of the slide is blocked by the retaining surface and an inner radial position distant from the retaining surface. Rotation of the drum creates centrifugal force that biases the slide toward the outer radial position.
According to a further embodiment of the present invention, an imaging drum assembly is provided for use in an imaging apparatus for forming images on sheet material. The imaging drum assembly comprises a rotatable imaging drum having an outer surface adapted to attract sheet material to the imaging drum. A material clamp is mounted to the drum and has a retainer positioning a retaining surface radially outward of the outer surface and forming a space therebetween. A slide is movable positioned within the space between an outer radial position where outward radial movement of the slide is blocked by the retaining surface and an inner radial position distant from the retaining surface. A biasing member urges the slide toward the outer radial position.